The present invention relates generally to the field of semiconductor processing equipment. More particularly, the present invention relates to methods and apparatus for generating plasma, for example coils, used with high density plasma deposition chambers. The methods and apparatus can be applied to other semiconductor processes, for example etch processes used to form integrated circuits.
One of the primary steps in the fabrication of modern semiconductor devices is the formation of a film, such as a silicon oxide film, on a semiconductor substrate. Silicon oxide is widely used as an electrically insulating dielectric layer in the manufacture of semiconductor devices. As is well known, a silicon oxide film can be deposited by a thermal chemical-vapor deposition (“CVD”) process or by a plasma-enhanced chemical-vapor deposition (“PECVD”) process. In a conventional thermal CVD process, reactive gases are supplied to a surface of the substrate, where heat-induced chemical reactions take place to produce a desired film. In a conventional plasma-deposition process, a controlled plasma is formed to decompose and/or energize reactive species to produce the desired film.
Semiconductor device geometries have decreased significantly in size since such devices were first introduced several decades ago, and continue to be reduced in size. This continuing reduction in the scale of device geometry has resulted in a dramatic increase in the density of circuit elements and interconnections formed in integrated circuits fabricated on a semiconductor substrate. One persistent challenge faced by semiconductor manufacturers in the design and fabrication of such densely packed integrated circuits is the desire to prevent spurious interactions between circuit elements, a goal that has required ongoing innovation as geometry scales continue to decrease.
Unwanted interactions are typically prevented by providing spaces between adjacent elements that are filled with a dielectric material to isolate the elements both physically and electrically. Such spaces are sometimes referred to herein as “gaps” or “trenches,” and the processes for filling such spaces are commonly referred to in the art as “gapfill” processes. The ability of a given process to produce a film that completely fills such gaps is thus often referred to as the “gapfill ability” of the process, with the film described as a “gapfill layer” or “gapfill film.” As circuit densities increase with smaller feature sizes, the widths of these gaps decrease, resulting in an increase in their aspect ratio, which is defined by the ratio of the gap's height to its depth. High-aspect-ratio gaps are difficult to fill completely using conventional CVD techniques, which tend to have relatively poor gapfill abilities. One family of dielectric films that is commonly used to fill gaps in intermetal dielectric (“IMD”) applications, premetal dielectric (“PMD”) applications, and shallow-trench-isolation (“STI”) applications, among others, is silicon oxide (sometimes also referred to as “silica glass” or “silicate glass”).
Some integrated circuit manufacturers have turned to the use of high-density plasma CVD (“HDP-CVD”) systems in depositing silicon oxide gapfill layers. Such systems form a plasma that has a density greater than about 1011 ions/cm3, which is about two orders of magnitude greater than the plasma density provided by a standard capacitively coupled plasma CVD system. Inductively coupled plasma (“ICP”) systems are examples of HDP-CVD systems. One factor that allows films deposited by such HDP-CVD techniques to have improved gapfill characteristics is the occurrence of sputtering simultaneous with deposition of material. Sputtering is a mechanical process by which material is ejected by impact, and is promoted by the high ionic density of the plasma in HDP-CVD processes. The sputtering component of HDP deposition thus slows deposition on certain features, such as the corners of raised surfaces, thereby contributing to the increased gapfill ability.
Even with the use of HDP and ICP processes, there remain a number of persistent challenges in achieving desired deposition properties. These include the need to manage thermal characteristics of the plasma within a processing chamber, particularly with high-energy processes that may result in temperatures that damage structures in the chamber. In addition, there is a general desire to provide deposition processes that are uniform across a wafer. Non-uniformities lead to inconsistencies in device performance and may result from a number of different factors. The deposition characteristics at different points over a wafer result from a complex interplay of a number of different effects. For example, the way in which gas is introduced into the chamber, the level of power used to ionize precursor species, the use of electrical fields to direct ions, and the like, may ultimately affect the uniformity of deposition characteristics across a wafer. In addition, the way in which these effects are manifested may depend on the physical shape and size of the chamber, such as by providing different diffusive effects that affect the distribution of ions in the chamber.
One particular challenge with HDP and ICP processes is the management of electric fields and voltages that arise from the use of radiofrequency (RF) coils that are used to generate plasma. The peak to peak voltage used to drive these coils can exceed one kilovolt (kV), and the associated effects of the prolonged use of this high voltage include chamber dome blackening, and particle and metal contamination of integrated circuits. Techniques used to reduce voltage and/or mitigate the effects of high voltage include external balanced capacitor banks, Faraday shielding and high voltage padding of the chamber dome surfaces. Although these mitigation techniques have provided at least some success to mitigate the effects of high voltages, improved techniques are sought.
In addition to the shortcomings described above, work in relation to the present invention also suggests that high voltages contribute, at least in part, to contamination of layers formed with HDP/CVD processes. The high voltages may cause the degradation of protective coatings, for example season coatings, applied to the inner surface of structures inside the chamber, such as a gas baffle, to prevent contamination. This degradation of the protective coating can cause contamination from the structures inside the chamber, for example metal contamination. Such contamination can effect the physical properties of the formed layers, for example the dielectric properties, of layers formed with HDP/CVD processes. As circuits continue to shrink, there is a need to provide layers with improved dielectric properties.
There is accordingly a general need in the art for improved systems for generating plasma that improve deposition across wafers in HDP and ICP processes.